The present invention relates to a theft prevention device for preventing the theft of goods, and more particularly, to a small-sized theft prevention device capable of preventing the theft of such goods with certainty and reliability.
A theft prevention device is typically composed of a tag which is attached to the protected goods and an electromagnetic wave projector installed at a gate or an entrance and exit of a shop for exciting the tag. With the arrangement of the prior art theft prevention device, when the goods having an attaching tag are passed through the gate, an alarm is sounded to thereby prevent the theft of the goods. There are proposed several systems of the prior art theft prevention device.
In a first system, the theft prevention device includes a tag which is attached to the goods and which is composed only of a resonator serving as an antenna, and an electromagnetic projector installed at the exit gate for exciting the resonator by emitting a powerful electromagnetic wave to the resonator and sounding the alarm upon reception of a reflected wave from the tag. It is possible to prevent the goods from being unknowingly carried out since the resonator of the tag resonates upon reception of the electromagnetic wave emitted by the electromagnetic wave projector and emits a reflected wave, and the electromagnetic wave projector sounds the alarm upon the reception of the reflected wave.
In a second system, the device includes a tag which is attached to the goods and which is composed of a resonator serving as an antenna, a receiver, a sound generator, and an electromagnetic projector installed at an exit gate for emitting a powerful electromagnetic wave to the resonator for exciting the resonator of the tag. It is possible to prevent the goods from being unknowingly carried out since the resonator of the tag resonates upon reception of the electromagnetic wave emitted by the electromagnetic wave projector when the goods are passed through the gate, whereby the receiver is operated to actuate the sound generator for generating a sound from the tag.
In a third system, the theft prevention device includes a tag which is attached to the goods and which is composed of a resonator serving as an antenna, a receiver, a transmitter, a radar device and an electromagnetic projector installed at an exit gate for emitting a powerful electromagnetic wave to the resonator for exciting the resonator of the tag. It is possible to prevent the goods from being unknowingly carried out since the resonator of the tag resonates upon reception of the electromagnetic wave emitted by the electromagnetic wave projector when the goods are passed through the gate, whereby the receiver is operated to actuate the transmitter for generating the electromagnetic wave which is detected by the radar device.
However, there are drawbacks in the prior art theft prevention device in that a powerful electromagnetic wave is required to be emitted from the electromagnetic wave projector installed in the exit gate for exciting the resonator of the tag. In a country where a Wireless Telegraphy Act prohibits the emission of such a powerful electromagnetic wave, the electromagnetic wave projector cannot be used.
According to the first to third systems of the prior art theft prevention device, although the tag is provided with an antenna also serving as a resonator for receiving the electromagnetic wave, the size of the antenna cannot be reduced.
To solve the drawbacks set forth above, the present inventor proposed a small sized theft prevention device which does not use an electromagnetic wave. The principle of this device resides in detecting light and vibration and sounding an alarm when the detected light and vibration meet a prescribed condition. However, there is no vibration sensor available which is suitably adapted for meeting the prescribed condition.